The global drive to replace the use of toxic lead metal and its alloys in industrial applications has focused, in part, on the development of new Pb-free solder alloys. In addition to the toxicity of lead, there are other problems concerning continued widespread use of inexpensive Sn--Pb and Pb-based solders. Current leaded solders lack shear strength and resistance to creep and to thermal-mechanical fatigue. A solder which exhibits enhancements of these properties and retains solderability is crucial in automotive and other heavy industry applications where the solder joints are subjected to many thermal cycles, severe vibrations, and sustained temperatures of up to 150 to 170 degrees C. The consequence of solder joint failure in critical applications where "lifetime" performance is now expected can be disastrous.
The excellent metallurgical wetting, or "solderability," of Sn--37%Pb (weight %) is thought to be promoted by the instantaneous formation of a thin layer of a very stable intermetallic compound at the molten solder/base metal interface. Interestingly, the interfacial intermetallic compound that aids solder wetting is always based on Sn (not Pb), e.g., Cu.sub.6 Sn.sub.5 forms at the interface between molten Sn--37%Pb solder and a Cu wire. The role of Pb in promoting solderability is much less understood, but seems related to its ability to strongly suppress the liquid surface tension of the solder alloy, lowering the contact angle of the molten solder which leads to better spreading and more interacting surface area for the solder joint to form. The eutectic solidification reaction of Sn--37%Pb also generates a highly refined mixture of Sn and Pb phases that produces unusual strength from rather weak constituents, i.e., Sn and, especially, Pb, along with good ductility for forming into wire and foil preforms. Thus, an effective alloy design strategy to develop a Pb-free solder is to start with Sn as the base of the alloy for metallurgical wetting, to add a second or third component to drive wetting and to depress the Sn alloy melting temperature, and to search for a composition that gives a highly-refined, eutectic-like solidification microstructure for an optimum balance of strength and ductility.
One additional design criteria for a Pb-free solder alloy intended for use in severe applications is to significantly improve on the ability of Sn--37%Pb to resist microstructural coarsening, thereby, retain strength and resisting metal fatigue even in high temperature, thermally cycled environments. A Sn--37%Pb solder joint solidification microstructure may start as a finely-spaced eutectic of Sn and Pb solid solution phases but can rapidly coarsen resulting in a lack of shear strength and resistance to creep and to thermal-mechanical fatigue. A new Pb-free solder should utilize microstructural design techniques that inhibit diffusion such as promoting intermetallic second phases formation to strengthen the Sn matrix instead of solid solution hardening or solidification of a finely dispersed primary phase like Bi. Another aspect of microstructural stability that should be addressed is the suppression of growth of the Cu--Sn intermetallic phase layers that initially perform a beneficial function for wetting. Unfortunately, after too much intermetallic growth, the interface between the solder and a Cu substrate can become a weak path for fatique crack growth. The real need is to develop new Pb-free solders that have similar processing characteristics and usage cost to Sn--Pb and Pb-based solders, but with improved mechanical properties and microstructural stability.
An important industrial consideration is the extensive investment in soldering equipment and manufacturing process design that is linked to existing leaded solders. This consideration favors the strategy of developing as close to a "drop in" Pb-free solder substitute as possible. In the electrical wiring and electronic packaging industries, a substitute is needed for Sn--37%Pb (wt. %) eutectic solder which melts at 183 degrees C. and is commonly used for a broad spectrum of electrically conductive joints. The melting point or liquidus temperature of a new Pb-free solder should be well below the range of adjustment, typically about 280 degees C. maximum, of commercial solder reflow ovens, wave and bath soldering units, and hand soldering guns intended for Sn--Pb solder to allow for a practical amount of superheat, typically 25 to 30 degrees C., during soldering.
One of the primary reasons for the popularity of Sn--37%Pb is its characteristic of excellent wettability and molten fluidity, or "solderability" when forming a solder joint on common metals, like Cu, steel, brass, and stainless steel. A eutectic solder alloy like Sn--37%Pb exhibits maximum fluidity as soon as melting begins because it has no "mushy" melting range. Effective soldering with Sn--37%Pb usually requires only a mild flux to remove surface oxides and to start the metallurgical solder wetting and bond formation in an ambient air environment. A large effort in the electronics industry to eliminate the use of CFC-based cleaning agents has lead to the universal push to ever milder fluxes that do not require post-reflow cleaning. Thus, any new Pb-free solder must be compatible with very mild fluxes. Also, any new Pb-free solder should not be sensitive to air oxidation.
The soldering needs of the heat exchanger industry, supplying automotive and industrial vehicle radiators, as well as industrial and residential climate control systems, and many other heavy industrial applications, such as hydraulic and pneumatic hose fittings connections, are perhaps broader and consume larger quantities of elemental Pb. This higher Pb consumption is because of the much broader use of Pb-based solders compared to Sn--37%Pb solder, especially, Pb--5%Sn solder, which melts gradually between 305 degrees C. and 316 degrees C. The Pb--5%Sn solder is commonly used for initial bath dip soldering of copper radiator cores and for some header and tank seams involving brass and steel. Sn--37%Pb is used only for secondary seams and finishing. The important solder properties in these applications are general mechanical durability, chemical stability, and thermal conductivity. The poor corrosion resistance and fatigue strength of Pb--5%Sn solder, particularly during pressurization cycles at temperatures of about 120 degrees C., are the primary property deficiencies that must be overcome by a Pb-free solder replacement. Solder alloy ingot and wire cost is a much more important criteria for heat exchanger manufacturing than for electronics, and any replacement for ultra-low cost Pb--5%Sn solder must be applied more efficiently, probably as a paste or preform foil, to compete on total manufacturing cost, even if significantly improved properties can be demonstrated.
A Pb-free termary eutectic solder alloy, Sn--4.7%Ag--1.7%Cu (weight %) is described in U.S. Pat. No. 5,527,628, issued Jun. 18, 1996, which exhibits a melting point of 217 degrees C. and very good solderability. This solder alloy liquid solidifies as a fine eutectic microstructure of Cu.sub.6 Sn.sub.5 and Ag.sub.3 Sn intermetallic phases dispersed in a Sn(bct) matrix when cooled under typical solder reflow conditions, a microstructure which is significantly stronger than that of Sn--Pb eutectic solder. A Sn--Ag--Cu ternary eutectic had not been reported in previous experimental or calculated phase diagram studies and initial test results show great promise for this alloy as a Pb-free solder. The only significant deficiency of the Sn--Ag--Cu eutectic solder appeared to be a susceptibility to intermetallic layer growth at the solder/Cu substrate interface, particularly at high aging temperatures, a feature that is shared by essentially all high-Sn, Pb-free solders.